The objective of this research is to contribute to the mechanistic understanding of how polycyclic aromatic hydrocarbons (PAHs) and halogenated aromatic hydrocarbons (HAHs) exert their toxicity. Human and wildlife populations are exposed to low levels of these ubiquitously distributed compounds and the long term health effect(s) resulting from exposure required further investigation. The aryl hydrocarbon receptor (AhR) is able to bind certain PAHs and HAHs in the cytosol and then dimerize with the aryl hydrocarbon receptor nuclear translocator protein (ARNT in the nucleus. The AhR-ARNT complex binds DNA regulatory elements upstream of the cytochrome P4501A1 (CYP1A1) gene and activates CYP1A1 expression. The phosphorylation status of AhR and ARNT has been shown to regulate their dimerization and subsequent activation of the CYP1A1 gene. Preliminary work in Dr. Perdew's lab has demonstrated that ARNT is exclusively phosphorylated on threonine residues. The hypothesis to be tested maintains that the phosphorylation status of ARNT regulates AhR-ARNT heterodimer function. To examine this hypothesis, the regulatory role of ARNT's predominant phosphorylation sites will be examined by following these aims: 1) Produce a phosphopeptide map for ARNT's phosphothreonine sites and identify the location of the phosphothreonine residues with the highest stoichiometry; 2) Assess the functional importance of the predominant phosphothreonine sites on ARNT by performing site-directed mutagenesis. This study will lead to a better understanding of how AhR function is regulated by ARNT's phosphorylation status.